Historically, the manufacture of rigid foamed polyurethanes has typically included the use of various combinations of a polyol, isocyanate, water, and trichlorofluoromethane (CFC-11) as a foaming or blowing agent. Traditionally, the materials were mixed together in a step-wise process. The polyol provided the polymer component which, when reacted with isocyanate, would polymerize and harden. Both polyol and isocyanate are liquid materials. The blowing agents traditionally used, such as CFC-11, are also liquid at room temperature but upon heating or undergoing rapid pressure reduction will volatilize. The addition of a blowing agent during the mixture process will create gaseous expansion upon either temperature elevation or pressure reduction within the polymerizing polyol matrix thereby causing the polyol to foam. Foamed polyurethanes have good mechanical and thermal insulating properties and show excellent dimensional stability, and chemical resistance. The foaming process allows the polyurethane to expand to fill a void defined by a formed structure such as a refrigerator door or body panel. Polyurethane foams are widely used as thermal insulating materials for home appliances, truck trailers and railroad cars, insulated storage vessels, building materials, and certain parts for automobiles.
It is well known in the art of foam production that use of liquid blowing agents is highly desirable. The reason is that foaming of the polyol is desired to occur as a final step in the process as the reactants are added to their mold. Generally, the blowing agent is added to polyol and mixed, then the blended mixture is subsequently added to isocyanate just prior to injection into a mold. Concerning the mixture of polyol to blowing agent, it has been common practice in those industries making polyurethane foams to use batch mixing process techniques to mix large volumes because of the relative insolubility of some common blowing agents with polyol. In batch mixing, the blowing agent is added to the foam polymer (preferably polyol) and continuously mixed by recirculating the mixture through a mixing reservoir over a lengthy period of time resulting, eventually, in a batch of mixed or, "blended," polyol and blowing agent. The time of mixing is directly related to the volume being mixed and the degree of difficulty of solubilizing the blowing agent into the polyol. Once the desired ratio of blowing agent and polyol was obtained and the materials thoroughly mixed, the process could be allowed to proceed to the polymerization and foaming steps by the addition of isocyanate and reduction of pressure or elevation of temperature. One aspect of realizing the solubility of some blowing agents with polyol was the necessity of keeping the blowing agent/polyol mix under constant high partial pressure of either an inert gas, such as nitrogen, or causing supersaturated conditions by pumping excess blowing agent into the mixing tank, the tank being kept in a closed loop isolated from and elevated relative to atmospheric pressure. For economic reasons, practitioners typically desired to make large quantity batches mixed well in advance of their needed use. The advance mixing and subsequent storage over a relatively long period of time allowed the use of common quality control techniques, such as sampling and weighing, to test the blended material and ensure compliance with specifications. Much of the mixing techniques were carried out on a trial and error basis. One significant problem of such mixing processes has been the retention of some blowing agents dispersed in the polyol. If, prior to use, the batches were found to be out of specification, the mixtures were reprocessed by adding the amounts of the various components to bring the blend within desired specifications. This process required remixing of the reagents with materials being routed back to mixing chambers. Such processes are inefficient, cumbersome and require additional and expensive equipment.
The prior art offers numerous examples of attempts at advancing the art of making polyurethane foam. Critical to the formation of foam is the amount and homogeneity of dissolved blowing agent. Properties of the quality of the foam will vary greatly depending on the amount, dispersion, and type of the blowing agent used. The present invention provides for an apparatus in which polyol and blowing agent may be mixed under highly controlled pressure levels and flow rates. A significant advantage of the present invention over prior art is the elimination of a need for batch mixing and the ability to monitor and control precisely both the amounts of reagents added together and the mixing thereof. The present invention allows for predetermination of all essential parameters of the materials used in the process of creating blended polyol giving reproducibility, predictability, and consistency in the foam formed from the blended polyol prepared by the present invention.
The present invention is patentably distinguishable from previous improvements in numerous respects. For example, U.S. Pat. No. 4,132,838 entitled, Process and Apparatus for the Preparation of a Reaction Mixture for the Production of Plastic Foams, by K. D. Kreuer et al., discloses an apparatus designed to obtain better control of mixing blowing gases with one of the reagents in order to achieve predictable homogeneity in the polyol/gas mixture. Kreuer's apparatus essentially controlled the velocity of flow rates of one of the reaction reagents into which a gas blowing agent was aspirated. Unlike the present invention which maintains the blowing agent in the liquid phase, Kreuer's device contemplated use of a vapor phase blowing agent, the addition of which to polyol was not under strict user control. Moreover, one embodiment of the Kreur system required return flow to a batch mixing chamber. Likewise, U.S. Pat. No. 4,157,427 entitled, Method for Entraining Gas in a Liquid Chemical Precursor for Reaction Injection Molding, by G. Ferber, disclosed an improvement in sampling the amount of vapor phase blowing agent infused into one of the reaction components. However, like other examples of the prior art, the apparatus contemplated recirculation of the reactant/gas mixture to a batch mixing chamber.
Various other improvements are found in the prior art which concern the mixing of a gas blowing agent. U.S. Pat. No. 4,288,230 entitled Method and a Device for the Production of a Solid-Forming or Foam-Forming Flowable Reaction Mixture, by W. Ebeling and V. Tennemann disclosed determining gas infusion by measuring quantitative flow rates of gas by taking density readings against volumetric flow. This device merely added gas and recirculated the gas/polyol mixture until the desired mix was obtained as determined by taking density and volumetric flow readings. In U.S. Pat. No. 4,376,172 entitled Closed Loop Control of Compressible Fluid Addition to a Mixture of such Fluid and a Liquid, by G. Belangee et al., a recirculation type system is disclosed that uses a plunger to measure the amount of gas blowing agent to be infused.
In U.S. Pat. No. 4,526,907 entitled Process and Device for the Preparation of a Reaction Mixture of at Least Two Components for the Production of Foams, by H. Thiele et al., another method of infusing gas blowing agent is disclosed for use in a closed loop system. Other patents such as, U.S. Pat. No. 4,933,115 entitled Process for the Production of a Flowable Mixture which Reacts to Form Foam from Flowable Components Stored in Storage Containers, by K. Krippl, and U.S. Pat. No. 5,000,882 entitled Apparatus for the Preparation of a Free-Flowing Mixture of Free-Flowing Components which Reacts to Form Foam, by F. Proksa, et al., disclose variations on the methodology of either addition of the blowing agent or the measurement thereof.
In addition to advances made in the art of infusing blowing agents, the art of foam production has had to make adjustments in the use of chemicals and gases for environmental and safety reasons. Based on theories that CFC compounds, particularly Freon, contribute to destruction of the protective ozone layer in the atmosphere, legislation has been passed in many countries regulating and/or prohibiting the manufacture or use of the traditional materials used as polyurethane blowing agents. The Clean Air Act of 1990 placed deadlines and restrictions on the use and manufacture of all CFC compounds. Compliance with legal restrictions requires the use of new methods and materials in the production of polyurethane foams, particularly regarding new blowing agents. Consequently a compelling need has arisen for new processes which will allow flexibility in selection of the blowing agent for the manufacture of polyurethane foams.
The polyurethane foam industry is in a state of transition. Traditional blowing agents have typically had relatively high boiling points with easily manipulated volatility making them relatively easy to use, mix, and store in the polyurethane production process. For example, CFC-11 or Freon, has a boiling point of approximately 75 degrees F at atmospheric pressure. The environmentally friendly blowing agents coming into use because of the recent legislation have not been so easy to use because of low boiling points and high volatility. For example, 1,1,1,2 tetrafluoroethane (HFC-134a) has a boiling point of approximately minus 15 degrees F. Moreover, some high boiling agents have exhibited low volatility requiring input of heat energy into the reaction process. Other blowing agents with very low boiling points have often exhibited properties which made their use problematic. For instance, hydrochloroflurocarbons have been shown generally to exhibit rapid vaporization giving rise to difficulty in keeping the material in solution during batch mixing as well as causing cooling of the reactant mixture with subsequent loss of expansion capacity during foam formation. Production equipment able to accommodate such new materials will require the ability to use blowing agents that have either a low or high boiling point. For example, Allied Signal manufactures more than 15 compounds, classified as refrigerants, with boiling points ranging from minus 126 to 117 degrees F, several of which have potential use as blowing agents. Moreover, the new processes will require equipment able to mix and blend components more quickly and more accurately than the batch mixing processes of the prior art. Where the process is to be adapted to existing polyurethane production systems, the new processes should be able to operate effectively with minimum redesign to existing systems.
Advancements in the art have begun to address these environmentally related problems. U.S. Pat. No. 5,055,272 entitled, Method for Producing Polyurethane Foam and Apparatus Therefor, by R. Wheeler et al., discloses an apparatus which is designed to use non-fluorocarbon gases and is also a single flow through system. However, unlike the present invention, Wheeler's device uses an expandable bladder that is pressurized by vapor phase gas and acts to maintain pressure for the gas as it is pumped into the liquid polyol. This type of device differs markedly from the present invention not only because of its use of vapor phase blowing agent, but also for the fact that the pressure within the system cannot bc accurately maintained due to the constant motion of contraction and expansion of the pump bladder. Thus, pressures in the system are continuously in flux rather than able to be maintained at a constant value. In U.S. Pat. No. 5,472,990 entitled Method and Apparatus for Nucleation of Polyurethane Foam which Results in Self-Adhering Microcellular Foam, by T. Craig et al., a single pass system is presented which is designed for using air as the blowing agent which is pumped into the polyol to create bubbles like the aforementioned art. The design of this system also relies on only one metering pump to pass the liquid reactants through the apparatus. Such a system makes accurate control of the quantities of each reactant difficult. In another example, U.S. Pat. No. 5,252,625 entitled Method for Producing Rigid Foams and Products Produced Therefrom, by A. McLaughlin, a device is disclosed which is able to utilize various blowing compounds but it incorporates a preblend mixing tank which also only uses a static mixer to infuse the gas with liquid component. Like the earlier batch mix apparatuses, there is little accuracy or control over the retention of blowing agent in the liquid phase or the measurement thereof. Finally, U.S. Pat. No. 5,444,100 entitled Method for the Mixing of Low-Boiling Foaming Agent, by M. Takezawa, discloses an apparatus designed to use environmentally safe blowing agents but the machine is specifically designed only to handle low-boiling point blowing agents. Moreover, just like the earlier prior art processes, this apparatus requires a recirculation gas/polyol mix tank.
The present invention addresses the drawbacks of previous advancements in the art by eliminating the need for recirculation batch mixing by providing an open-loop, single pass, "on demand" system, that is, a process system that can accurately mix and blend specified amounts of liquid reagents (polyol and liquified blowing agent), monitor the amounts of each reactant before and after blending, and present the blended mixture with known composition directly to a foaming extrusion head (e.g. no remixing tank is necessary) or to a day use tank storage under conditions which will allow maintenance of known reactant ratios. Because of the high degree of versatility in the manner in which reaction components may be added and monitored, the present invention also addresses the difficulties associated with legislative requirements by allowing various types of environmentally-friendly chemical blowing agents to be employed. Moreover, the versatility allows the device the capacity for accommodating both high and low-boiling point blowing agents. Concerning low boiling point blowing agents, a preferred embodiment of the present invention is use of 1,1,1,2 tetrafluoroethane (HFC-134a). This compound is known to be highly insoluble in polyol and has been used by others wherein a solubilizing agent, such as dimethyl ether (DME) was necessary (U.S. Pat. No. 5,409,962 entitled Substantially Constant Boiling Blowing Agent Compositions of 1,1,1,2 Tetrafluorothane and Dimethyl Ether by P. L. Bartlett and J. A. Creazzo) to cause acceptable solubilization of the HFC-134a in the polyol. It is known that very small amounts of some solubilizing agents, such as DME, can have dramatic effects on enhancing the solubility of such compounds as HFC-134a in polyol. However, use of such solubilizing agents may cause problems with the polymerization process and the use of such agents therefore should be avoided where possible. A major advantage and a preferred embodiment of the current invention is the ability of solubilizing HFC-134a into polyol in a pure form without the need for any solubilizing substance.